Prior to setting forth a short discussion of the related art, it may be helpful to set forth definitions of certain terms that will be used hereinafter.
The term “MIMO” as used herein, is defined as the use of multiple antennas at both the transmitter and receiver to improve communication performance. MIMO offers significant increases in data throughput and link range without additional bandwidth or increased transmit power. It achieves this goal by spreading the transmit power over the antennas to achieve spatial multiplexing that improves the spectral efficiency (more bits per second per Hz of bandwidth) or to achieve a diversity gain that improves the link reliability (reduced fading), or increased antenna directivity.
The term “beamforming” sometimes referred to as “spatial filtering” as used herein, is a signal processing technique used in antenna arrays for directional signal transmission or reception. This is achieved by combining elements in the array in such a way that signals at particular angles experience constructive interference while others experience destructive interference. Beamforming can be used at both the transmitting and receiving ends in order to achieve spatial selectivity.
The term “beamformer” as used herein refers to RF and/or digital circuitry that implements beamforming and includes combiners and phase shifters or delays and in some cases amplifiers and/or attenuators to adjust the weights of signals to or from each antenna in an antenna array. Digital beamformers may be implemented in digital circuitry such as a digital signal processor (DSP), field-programmable gate array (FPGA), microprocessor or the CPU of a computer to set the weights (phases and amplitudes) of the above signals. Various techniques are used to implement beamforming such as using a Butler matrix, Blass Matrix, Rotman Lens and/or phased array of antennas. In general, most approaches to beamforming attempt to provide simultaneous coverage within a sector using multiple beams.
WiFi has been implemented with a limited amount of frequency resources that use techniques of collision avoidance to allow multiple user equipment (UEs) to share the same channel. As the numbers of UEs proliferate, the impact of such a scheme restricts the ability of a base station Access Point (AP) to support many users without impacting the performance to and from each. This invention discloses an apparatus and methods to allow the reuse of resources by implementing AP clusters using multi-beam antennas breaking down a sector area of coverage into smaller subsectors. In order to accomplish this, several limitations of multi-beam antennas must be addressed. First, since WiFi is a time division multiplex system (TDD), the transmitting and receiving functions use the same channel. Unsynchronized operation between APs means a transmitting AP's signal may interfere with the reception of another AP that uses the same channel unless sufficient isolation (e.g., 125 dB) is provided between the transmitting and receiving functions. Prior art cited above addresses the problem by using physically separated antenna arrays for transmit and receive and by providing cancellation of each transmitted signal within the receiver processing functions. Another limitation is that multi-beam antennas do not offer infinite separation of the coverage of one beam to the others. The following discusses the impacts of this performance limitation and presents approaches to mitigate its effect.